1. Field of the Invention
The present invention relates to a motor having improved machinability so that it can be made very compact.
2. Related Background Art
As a conventional compact motor, for example, a compact cylindrical stepping motor shown in FIG. 12 is available. A stator coil 105 is concentrically wound on each bobbin 101. The bobbin 101 is sandwiched and fixed by two stator yokes 106 in the axial direction. Stator teeth 106a and 106b are alternately arranged in each stator yoke 106 in the circumferential direction of the inner-diameter surface of the bobbin 101. The stator yoke 106 which is integral with the stator teeth 106a or 106b is fixed to a case 103, thus constituting each stator 102.
A flange 115 and a bearing 108 are fixed on one of the two cases 103, and the other bearing 108 is fixed to the other case 103. A rotor 109 is constituted by a rotor magnet 111 fixed to a rotor shaft 110. The rotor magnet 111 forms a radial gap portion together with the stator yokes 106 of the stators 102. The rotor shaft 110 is rotatably supported between the two bearings 108.
In the conventional compact stepping motor described above, however, since the cases 103, the bobbins 101, the stator coils 105, the stator yokes 106, and the like are concentrically arranged around the rotor 109, the outer size of the motor is undesirably increased. Since the magnetic flux generated by energization of the stator coils 105 mainly passes through an end face 106a1 of the stator tooth 106a and an end face 106b1 of the stator tooth 106b, as shown in FIG. 13, it does not effectively act on the rotor magnet 111. Therefore, the motor output is not increased.
The present applicant proposes a motor, in which these problems are solved, in U.S. patent application Ser. No. 08/831,863.
In this motor, a rotor made of a permanent magnet which is equally divided in the circumferential direction to be alternately magnetized to different poles is formed into a cylindrical shape. The first coil, the rotor, and the second coil are sequentially arranged in the axial direction of the rotor. The first outer and inner magnetic poles excited by the first coil are set to oppose the outer and inner circumferential surfaces, respectively, of the rotor, and the second outer and inner magnetic poles excited by the second coil are set to oppose the outer and inner circumferential surfaces, respectively, of the rotor. A rotating shaft serving as the rotor shaft extends from the inside of the cylindrical permanent magnet.
The motor having the above arrangement has a high output and its outer size can be made small. However, since the first and second inner magnetic poles have small diameters, it is difficult to machine their magnetic pole teeth. Since the rotating shaft extends from the inside of the cylindrical permanent magnet, it is difficult to machine the rotor having the rotating shaft.
For these reasons, recently, the present applicant has proposed a motor, in which the inner magnetic poles have shapes that can be machined well, in U.S. Patent Application the serial number of which is not yet assigned. In this proposed motor, the rotor has rotating shaft that can also be machined easily.